Allowance for Loan Losses - Composition of Nonaccrual Loans by Class (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable Impaired [Line Items]
Increase in non-performing loans	$ 3,766	$ 2,174
Real Estate - Commercial [Member]
Financing Receivable Impaired [Line Items]
Increase in non-performing loans	2,092	273
Other Real Estate Construction [Member]
Financing Receivable Impaired [Line Items]
Increase in non-performing loans		47
Real Estate - Residential [Member]
Financing Receivable Impaired [Line Items]
Increase in non-performing loans	1,341	1,496
Home Equity [Member]
Financing Receivable Impaired [Line Items]
Increase in non-performing loans	247	218
Commercial Loan [Member]
Financing Receivable Impaired [Line Items]
Increase in non-performing loans	58	54
Consumer Loans [Member]
Financing Receivable Impaired [Line Items]
Increase in non-performing loans	28	86
Loans 90 Days or More Past Due [Member]
Financing Receivable Impaired [Line Items]
Increase in non-performing loans	3,064	1,044
Loans 90 Days or More Past Due [Member] | Real Estate - Commercial [Member]
Financing Receivable Impaired [Line Items]
Increase in non-performing loans	2,092	273
Loans 90 Days or More Past Due [Member] | Other Real Estate Construction [Member]
Financing Receivable Impaired [Line Items]
Increase in non-performing loans		47
Loans 90 Days or More Past Due [Member] | Real Estate - Residential [Member]
Financing Receivable Impaired [Line Items]
Increase in non-performing loans	805	606
Loans 90 Days or More Past Due [Member] | Home Equity [Member]
Financing Receivable Impaired [Line Items]
Increase in non-performing loans	$ 167	$ 118